Log a warning on parse errors

Fixes #62

CHANGELOG.md

@@ -10,6 +10,10 @@ Expanded filename associations, so difftastic recognises more files.
 
 Improved word diffing on punctuation in comments.
 
+When logging is enabled (e.g. `RUST_LOG=warn`), difftastic now warns
+on syntax errors. Difftastic is intended to be robust against syntax
+errors, so this is primarily intended for parser debugging.
+
 ## 0.12
 
 ### Display

src/tree_sitter_parser.rs

@@ -447,6 +447,13 @@ fn syntax_from_cursor<'a>(
 ) -> &'a Syntax<'a> {
     let node = cursor.node();
 
+    if node.is_error() {
+        let position = nl_pos.from_offsets(node.start_byte(), node.end_byte());
+        let content = &src[node.start_byte()..node.end_byte()];
+
+        warn!("Tree-sitter syntax error at {:?}: {}", position.get(0), content);
+    }
+
     if config.atom_nodes.contains(node.kind()) {
         // Treat nodes like string literals as atoms, regardless
         // of whether they have children.